<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        * {
            margin: 0px;
            padding: 0px;
        }
        body {
            background-image: url(images/1.jpg);
        }
        #mask {
            background-color:  rgba(255,255,255,0.3);
            height: 200px;
            text-align: center;
        }
        #mask img {
            width: 200px;
            margin-top: 35px;
            cursor: pointer;
        }
    </style>
</head>
<body>
<div id="mask">
    <img src="images/1.jpg" alt="">
    <img src="images/2.jpg" alt="">
    <img src="images/3.jpg" alt="">
</div>

<script>

    var imgArr = document.getElementsByTagName("img");
    var bd = document.body;
    for(var i=0;i<imgArr.length;i++){
        imgArr[i].onclick = function () {
            bd.style.backgroundImage = "url("+this.src+")"
        }
    }
</script>

</body>
</html>